Extracts of Klebsiella aerogenes inactivate glutamine synthetase. The reaction has characteristics similar to the cytochrome P-450-dependent mixed function oxidations. Ascorbic acid has been used in model systems of these oxidations. Ascorbic acid rapidly inactivates glutamine synthetase with characteristics of the Klebsiella extract: (1) it is oxygen-dependent; (2) Catalase blocks the reaction, implying participation of hydrogen peroxide; (3) Ferric salts stimulate inactivation while chelating agents prevent inactivation. Of the four amino acids which are feedback inhibitors of glutamine synthetase, only histidine affected the ascorbate-mediated inactivation. Histidine could completely protect the enzyme. A Kd for histidine was estimated from the protective effect (0.26 mM). The substrates glutamate and ATP had differential effects on the two forms of the enzyme. Unadenylylated enzyme was protected from inactivation, while the adenylylated enzyme became more susceptible to inactivation.